In short, you need a UPS that will support whatever you want to connect to it for as long as you would like it to run.
It is always best to allow a little overhead when sizing a UPS, you never know what else you might like to add to your system later on. The only truly accurate method of determining the amount of power required is to connect a power meter to the equipment you wish to protect. This is rarely convenient so many manufacturers provide a UPS Size Calculator where you can enter in the details of your equipment and the calculator will recommend a UPS based on the specifications of your equipment.
As a rule of thumb, for personal PC's allow 200W per PC, 50W per monitor and 20W per external device such as modems, routers, hard drives. Once you have a figure allow plenty of headroom, so if you come up with 300 Watts, dont buy a 320 Watt UPS, look at around 500 Watts.
For Servers, allow about 500-800 Watts per server and 100 Watts per network switch. This is only a rough guide but it gives you a quick idea of the capacity you might require. Obviously the servers and switches wont always be drawing this amount of power when lightly loaded.
